X-ray Photoelectron Spectroscopy (XPS) for Surface Analysis
Albert Einstein was the most famous scientist that ever lived. He is, of course, known for his , which is part of his Theory of Relativity. It is not surprising that Einstein has won the Nobel Prize in Physics, but he didn’t get it for his Relativity Theory. At the time, it was far too controversial for a Nobel Prize. Instead he got it for the discovery of the Photoelectric effect.
Light can be seen as a wave, but also as a stream of particles, so called photons. Einstein discovered that when a particle a photon hits an electron, the photon “disappears” and gives all its energy to the electron. In a material, electrons are bound to atoms with a binding energy. If the energy of the photon was higher than the binding energy of the electron then the electron has so much energy that it can escape from the atom that it belongs to. The solar panels that you might have on the roof of your house are developed based on this principle: since the electrons that escape are prevented from turning back, they can escape only in one direction and, when the solar panel is connected, an electric current starts to run. It removes the escaped electrons on one side. On the other side it supplies the solar panel with the electrons to fill the holes that were made. A solar panel is an “electron pump”.
The energy of photons is determined by the wavelength (color) of the light: The shorter the wavelength of the light, the higher the energy of the photon. X-rays have a very short wavelength, and X-ray photons have a high energy. The surface analysis technique XPS (X-ray Photoelectron Spectroscopy) uses the photoelectric effect to analyze the surface of samples.
In XPS analysis, an X-ray with a precisely known energy (wavelength) is aimed at the surface. The X-ray photon gives its energy to an electron in the material. This energy is so high that the electron escapes from its atom. To escape from the atom has to lose the binding energy that kept him in the atom. The remainder of the energy is kinetic energy: the velocity of the electron.
Since we know the energy of the X-ray photon, and we can measure the energy of the electron (by measuring its velocity), we know that the difference between the two must be the binding energy: The energy that is needed to make the electron escape from the atom. This Binding energy is characteristic for the chemical element.
So, why is XPS analysis a surface sensitive analytical technique? The X-rays are going deep into the material, but the electrons that are emitted from the atoms can only travel a few nanometers without losing energy. The largest part of the signal in an XPS spectrum is the background from electrons that have lost energy. But on top of this background, there are peaks at the exactly correct binding energies for certain elements. This means that those elements must have been present at the surface of the sample.
One of the nice features of XPS analysis is that it is a quantitative technique: It does not only tell what elements are present at the surface, but also how much of it is there.